The present invention relates to an antenna system including a radiowave concentration means, like a reflector, a lens or the like, and a primary feed antenna, which is located at a focal point, where incoming radiowave beams are concentrated.
It is generally known, to use antenna systems, which include a parabolic reflector and a feed horn provided at the focal point of the parabolic reflector, for receiving radiowave signals.
From U.S. Pat. No. 4,742,359 it is known, that said feed horn can be replaced by a helical antenna with two ends whereby the first end is linked to a feeder line. For the purposes of the following explanation it is understood that the said feeder line is aligned with the axis of the said helical antenna. Such a helical antenna may be built as a so-called endfire helical antenna, where under maximum received power conditions the direction of the signal power flow at the said first end is in the same direction as the received radiation. Such a helical antenna can also be built as a so-called backfire helical antenna, where under maximum received power conditions the direction of the signal power flow at the said first end is in the opposite direction to the received radiation.
In the said U.S. patent an antenna system is presented, which comprises a reflector, a primary helical antenna having a coil with a pair of ends, said coil located at the focal point of said reflector so that the axis of the helical antenna coincides essentially with the axis of said reflector. A feeder line couples the antenna system with an external circuit, so that primary helical antenna represents a backfire helical antenna coupled with said feeder line at the nearer end from said reflector and the other end of the helical antenna is free standing, and said feeder line is a coaxial cable.
A typical semi-rigid coaxial cable has an insertion loss of 1.5 dB/m at a frequency of 12 GHz, which is used for current direct reception of satellite TV-signals. In systems, which are state of the art, a length of nearly 8.1 meter is required, for a reflector of a diameter of 40 centimeter, resulting in a total cable loss of nearly 0.15 dB. This value adds directly to the noise figure of the antenna system (typically less than 1.4 dB) and will be substantially higher at higher frequencies, such as the 22 GHz band proposed for future satellite TV systems.
It is an object of the present invention to provide a compact antenna system, for receiving electromagnetical, preferably microwave, signals, where the use of a feeder line for microwaves between a primary feed antenna and external circuits can be reduced to a great extent or even be avoided.
The antenna system according to the present invention includes a concentration means, such as a reflector, e.g. parabolic, or a microwave lens, e.g. Luneburg-like. The said concentration means concentrates received microwave beams at one focal point or at several focal points respectively and at each of these focal points a primary feed is provided, which is supported by a hollow structure, which may be tubular, circular, rectangular, or the like. This structure houses electronic means, e.g. a low noise converter (LNC), which convert, filter and/or amplify signals received by the said primary feeds.
By arranging the electronic means inside the tubular structure the use of expensive feeder lines, such as a semi-rigid coaxial cable, can be reduced to a great extent or even be avoided. Additionally respective links or connectors can be avoided. The antenna system according to the invention allows fewer mechanical parts, a lighter weight, and reduced costs relative to the prior art.
Additionally insertion losses of such a cable can be reduced or avoided respectively, whereby the noise figure will be improved and the performance of the antenna system can be increased.
If the tubular structure is provided with an adjustment mechanism the feed position can be changed to suit concentration means with different focal points, e.g. by the use of reflectors of different diameters.
The use of helical coils has the advantage that they can be changed very easily, whereby the reception of signals with right-hand or left-hand circular polarization is possible.
The use of backfire helical antennas has the advantage that such an antenna system is quite compact.
If parts of the said electronic means are integrated and realized as part of an integrated circuit, e.g. as Monolithic Microwave Integrated Circuit, or as part of a hybrid circuit, space and more costs can be saved.
As especially in known systems using a microwave reflector, the quantity of feeder line needed is high, this invention can preferably replace such systems.